1. Field of the Invention
The present invention relates to a starter for an internal combustion engine having an electric motor that is driven to compulsory rotate an output shaft of the engine and start the engine.
2. Description of the Related Art
Generally, a starter for a vehicle internal combustion engine has a power transmission mechanism for transmitting the driving force of the electric starter motor to the engine output shaft such as a crankshaft. A typical structure of such power transmission mechanism is described as follows (See JP-A-2004-169668).
The power transmission mechanism includes an engine-side power transmission member, an electric-motor-side power transmission member, and a one-way clutch. The engine-side power transmission member is fastened to the engine output shaft with bolts and rotates with the engine output shaft. The electric-motor-side power transmission member is provided rotatably relative to the engine output shaft, while being coupled to a rotational shaft of the electric motor. The one-way clutch is located between the engine-side power transmission member and the electric-motor-side power transmission member. The one-way clutch permits torque transmission only in one direction between these members, that is, the direction from the electric-motor-side power transmission member to the engine-side power transmission member (more specifically, from the electric motor to the engine output shaft).
In the starter for an internal combustion engine having such power transmission mechanism, when the electric motor is driven on request for engine start-up, the one-way clutch is brought into engagement to transmit torque from the electric motor to the engine output shaft. When combustion is completed in the internal combustion engine, in other words, when the internal combustion engine starts rotating autonomously without the aid of the electric motor, the one-way clutch is released. This results in disconnection between the engine output shaft and the electric motor.
In the starter, the interior of the power transmission mechanism is partitioned by the engine output shaft, the engine-side power transmission member and the electric-motor-side power transmission member. Lubricant is supplied from an engine body to the interior of the power transmission mechanism for lubricating the power transmission mechanism. A sealing member is provided respectively between the engine-side power transmission member and the electric-motor-side power transmission member, and between the engine body and the electric-motor-side power transmission member. The sealing member prevents lubricant from leaking out of the interior of the power transmission mechanism.
In contrast, no specific sealing treatment for preventing lubricant leakage is applied to a certain portion of the aforementioned power transmission mechanism, where the engine-side power transmission member is mounted to the engine output shaft. However, in the structure of the power transmission mechanism, the engine-side power transmission member is fastened to the engine output shaft, and the pressure between their contact surfaces is thus increased. Therefore, this structure hardly allows lubricant to leak from the mounted portion of the engine-side power transmission mechanism, that is, between the engine output shaft and the engine-side power transmission member. Nonetheless, additional sealing treatment may be applied between the engine output shaft and the engine-side power transmission member for more reliably sealing the lubricant within the interior of the power transmission mechanism.
Such additional sealing treatment provided for the foregoing mounted portion can be a sealing member, such as a gasket and an O-ring. This enhances sealing performance of the mounted portion, and accordingly, sealing performance of the power transmission mechanism.
However, the engine-side power transmission member is designed to transmit torque to the engine output shaft large enough to compulsory rotate the shaft at engine start-up. Thus, the engine-side power transmission member needs to be coupled tightly to the engine output shaft in order to maintain high reliability of the power transmission mechanism. Therefore, applying the aforementioned sealing treatment causes the sealing member to excessively deform as the engine-side power transmission member is fastened to the engine output shaft. This does not ensure sealing performance high enough for the mounted portion of the engine-side power transmission mechanism. Thus, to obtain the sufficient sealing performance, the engine-side power transmission member should be fastened to the engine output shaft with a smaller force. As a result, a higher coupling strength between the engine output shaft and the engine-side power transmission member is traded off for higher sealing performance.